My pre instructions are;
* 100% important issues;
1. Don't turn away from the FOP, interact with the crowd etc.
2. Don't chase or stop the ball going out of bounds.
(Believe it or not, I don't skip this phase anymore. I got assigned two A/R's from a tier above me this season and I thought they'd know their job as they're on the leagues, and, guess what? One of them went into the stands to fetch the ball from a throw-in... So, yeah, cover it.)
* What you want them to do regarding nets/toss (I've been advised, nets then toss, but everyone varies.)
* Cover the main KMI basics;
First incidents you're going to deal with is what? 90% of the time it's a ball in or out of play; So, cover the zones you want to operate in, cover what you do if you don't know who has touched it last, cover the communication between the two of you. And lastly, cover what happens in the event you both end at cross-purposes.
From there, it's an easy jump to fouls/misconducts -> penalties -> Offside calls and then finish with who records what and that's it.
Much as I prefer to be brief these days, those are the key stuff you want to cover, and it is up to you how you want to operate on the day. I would keep to the three zones method ~ old fashioned as it is, it works 9/10.
Finally; Don't overcomplicate it. You don't need to cover 10000000% of every scenario, like a maths book with a gentleman buying 59 watermelons and three coca cola bottles, so how many watermelon flavoured coke glasses is he producing? Just stick to the basics, you'll do fine.